Characteristics of Platyelminthes. cxli 



state, never possess any dig-estlve tract ; Ampldptyches urna, which 

 appears to be the connecting* link between the Trematodes, to which 

 order it belong-s, and the Ccstodes, is also devoid of this system, 

 and dependent upon imbibition through the external integument 

 for nutriment. In the Trematodes, and Turhellaria, with the 

 exception of the Nemertinea in which there is a perivisceral cavity, 

 the walls of the digestive tract are not separable ordinarily as 

 distinct layers from the rest of the parenchyma, except so far as the 

 contents of the hepatic cells, which clothe the interior of the system, 

 may enable us to distinguish them. In the Trematodes, the 

 digestive tract consists either of a simple stomachal coecum, or 

 of two similar structures, appended to a short muscular pharynx ; 

 and the two coeca may either each end blindly, or may anastomose, 

 or may give off great numbers of lateral ramifications. The Tre- 

 matodes are never proctuchous ; and in this those Turhellaria which, 

 from their possession of a multi-i'amified digestive apparatus, are 

 called ' Dendrocoelous,'' resemble them. The ' Hhabdocoelous ^ Tur- 

 hellaria may be either proctuchous or aproctous. The Nemertine 

 Turhellaria, which are called ^ Rhynchocoelous,^ from possessing 

 a proboscis armed with a calcareous style, and lodged in a tube 

 distinct from the digestive canal, are always proctuchous. Their 

 digestive tract takes a straight antero -posterior course, but is 

 provided with lateral sacculations. The Nemertinea differ further 

 from the other Platyelminthes in possessing a pseud-haemal vas- 

 cular system, which consists of a dorsal and two lateral vessels 

 connected wath each other in the neighbourhood of their nerve- 

 ganglia. All Platyelminthes possess a water-vascular depuratory 

 system, which opens externally either by a single posteriorly placed 

 pulsatile vesicle, or by two symmetrical orifices more anteriorly 

 placed. Its walls are often of different characters in various parts, 

 being contractile towards their outlets, and possessed of vibratile 

 cilia in their peripheral ramifications, in which again the calcareous 

 concretions already spoken of are often observed to be contained. 

 The flatness of their bodies, which enables aeration to be so readily 

 and thoroughly effected in the most of the free and in the ento- 

 parasitic rej)resentatives of this class ; and the multi-ramified 

 character of the digestive and depuratory systems, enable the 

 Platyelminthes to dispense with specialized respiratory and circu- 

 latory organs, the more or less cylindriform Nemertinea alone 



